Adding Bugli model

[mcolomermolla]

• [x] Update ccsn.py and ccsn_loaders.py
• [x] Update registry and models tests
• [x] Jupyter doc

[JostMigenda]

Thanks for the changes; this is now looking quite good!

Regarding the test failures:

=========================== short test summary info ============================
FAILED python/snewpy/test/test_01_registry.py::TestModels::test_Bugli_2021 - KeyError: 'Bugli_2021'
FAILED python/snewpy/test/test_02_models.py::TestModels::test_Bugli_2021 - FileNotFoundError: /home/runner/.astropy/cache/snewpy/models/Bugli_2021/hydro_3d_snewpy_average
===== 2 failed, 62 passed, 26 deselected, 11 warnings in 155.06s (0:02:35) =====

• [x] Add Bugli_2021 to the models/model_files.yml
• [x] Update the ccsn repo URL (at the top of that file) to v0.3 (instead of v0.2)—that version includes the new model files, so the tests should work

Also, before we merge this we still need a demo notebook.